Prevention of well casing corrosion



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3 fail/77mg J2, PM-.1 0. Mfizkv Nov. 6, 1956 P. e. NAHIN ET AL PREVENTION OF WELL CASING CORROSION Filed July 31, 1952 United States Patent 2,769,921 PREVENTION OF WELL CASING CORROSION Paul G. Nahin, Brea, and Earle R. Atkins, Jr., Whittier, alif., assignors to Union Oil Company of California, Los Angeles, Calif., a corporation of California Application July 31, 1952, Serial No. 301,901 4 Claims. (Cl. 250-106) This invention relates to a method for preventing corrosion of well casings, and in particular concerns a method for preventing the type of corrosion which is brought about by bacterial or biochemical action.

It is known that in certain geographical locations a large proportion of the corrosion which occurs in buried iron pipes is brought about by bacterial or biochemical action. Such corrosion is in some instances indirectly caused by the so-called sulfur-producing bacteria, such as Vibrio desulfuricans, which exist under anaerobic conditions and reduce naturally-occurring sulfates to hydrogen sulfide which attacks iron to form iron sulfide. In other instances it may be brought about more directly by the presence of so-called iron bacteria which promote the attack of acid waters on iron by continuously consuming the iron salts formed thereby. In many oil-fields the well bores traverse clay formations which contain large quantities of sulfate in the form of gypsum. They may also traverse brine or slightly acid water strata, and penetrate deep into the earth where anaerobic conditions prevail. Consequently the opportunities for bacteriallyinduced corrosion of the iron casing which lines the bore are very great, and in many oil-fields the problem of casing corrosion presents serious economic and operational difficulties. Means for preventing such corrosion based on conventional methods for destroying the bacteria at fault, e. g., through the use of liquid bactericidal compositions, are for the most part ineffective by reason of the difiiculty encountered in placing and maintaining the bactericide in the long narrow annulus between the casing and the walls of the bore.

It is accordingly an object of the present invention to provide a means for preventing bacterially induced corrosion of well casings.

Another object is to provide a simple and eifective means for destroying corrosion-inducing bacteria in the vicinity of well bores.

A further object is to provide a means for destroying corrosion-inducing bacteria in the vicinity of cased-in well bores without removing the casing from the bore.

Other objects will be apparent from the following description of the invention, and various advantages not specifically referred to herein will occur to those skilled in the art upon employment of the invention in practice.

We have now found that the above related objects may be realized through the use of gamma rays. More particularly, we have found that corrosion-inducing bacteria which are present in the vicinity of well bores may be completely destroyed by passing through the bore a source of relatively high intensity gamma radiation. The gamma rays are highly penetrating and readily pass through the casing and for a considerable distance into the earth surrounding the bore. Accordingly, the use of gamma radiation in accordance with the invention destroys not only the bacteria which are within the casing but also those present behind the casing and even those present within the formations traversed by the bore. As a result, a single gamma ray treatment according to the invention will serve efiectively to prevent bacterially-induced corrosion over a long period of time, and such treatment may be eflected without removing the casing. In some instances it is not even necessary to remove the well tubing, although the treatment is usually most conveniently carried out at times when the tubing is pulled for periodic cleaning. Gamma rays characteristically do not induce secondary radiation in the materials through which they pass. Accordingly, use of gamma radiation in accordance with the invention does not involve any danger of the oil pro- 7 duced by the well or any of the well equipment, e. g. the

casing, tubing, etc., becoming radioactive.

However, it is necessary to protect those who handle the gamma ray source from the intense radiation thereof.

In the accompanying drawing which forms a part of this specification, Figure 1 is a diagrammatic view of a a typical cased well bore being treated in accordance with the invention. Figure 2 diagrammatically illustrates in cross-section a suitably shielded container for the gamma ray source.

Referring now to Figure 1, well bore 11 is shown penetrating the surface of the earth and extending through various subterranean strata 12, 13 and 14 into an oilproducing formation 15. Well casing 16 lines bore 11 for a portion of its length, being secured to the walls of the bore at its lower end by cement plug 17. The conventional well head or Christmas Tree and associated equipment, including the well tubing, has been removed and the upper portion of the radiation shield shown in Figure 2 has been positioned over the upper end of casing 16. Cable 18 is attached at one end to the gamma ray source container 19 and passes over pulley 20 to be attached at its other end to a winch or the like, not shown. By suitable operation of the winch the gamma ray source container can be lowered into and withdrawn from the well bore at will. As the gamma ray source traverses the bore the intense radiation passes through the casing and penetrates for a considerable distance into the subterranean formations traversed by the bore, thereby destroying all corrosion-inducing organisms in the immediate vicinity of the bore. The rate at which the gamma ray source is caused to traverse the bore will depend upon the intensity of the radiations emanating from the source. With gamma ray sources now available from fission products such rate may vary from as little as to as high as 6000 ft./hr.

Figure 2 represents a cross-sectional view of a shielded container for the gamma ray source. Said container comprises a base plate 31 which may be square or circular in shape and which is construted of a material resistant to gamma ray penetration, e. g. lead or thick concrete. Shield 32 is attached to base plate 31 by means of bolts 33 and comprises a block of gamma ray absorbing material having an internal cavity 34. Ring bolts 35 extend from the top of shield 32, and serve as means for attaching lifting means to the assembly. A relatively small central passage 36 communicates between the top surface of shield 32 and cavity 34, and is closed by a sliding plate 37 hinged at one side by pin 38. The gamma my source is contained in an inner container 39 which occupies cavity 34 and is fitted with a ring bolt 40. When the shielded container is assembled as shown, protection from the gamma radiation is afforded by the thick walls of shield 32 and base plate 31. Hinged plate 37 prevents radiation from escaping through passageway 36. When it is desired to treat a well bore in accordance with the invention, bolts 33 are loosened so that shield 32 may be detached from base plate 31. Plate 37 is swung aside and cable 18 which is attached to the well hoist is passed through passageway 36 and hooked onto ring bolt 40. Cables are attached to ring bolts 35 and run to a crane or other hoisting mechanism. While maintaining tension 3 on cable 18 in order to keep container 39 within the cavity of shield 32, the shield is hoisted directly over the Well casing. By suitably operating the well hoist, container 19 may be lowered into the well and withdrawn as desired. Upon completion of the treatment, container 19 is drawn up into cavity 34, and shield v32. is hoisted back onto base plate 31 and is secured thereto by replacing bolts 33.

The gamma radiation source must be capable of providing sufficient gamma radiation to pass through the well casing and penetrate into the formation, and should be capable of providing a gamma ray intensity of at least one curie per foot of source. As previously stated, the speed with which the source is passed through the bore varies directly with the strength of the source, and accordingly it is usually advantageous to employ a source capable of providing as high a radiation intensity as possible, e. g., as high as 100,000 curies per foot of source. A number of radioactive isotopes, e. g., cobalt-60, zinc-65, cesium-134, tantaluml 82, etc. are capable of providing sufiiciently high intensity gamma radiation to be employed in practice of'the invention, but the most convenient source material comprises mixed or separated uranium fission products obtained as by-products from nuclear reactors or piles.

As will be readily apparent to those skilled in the art, various procedures and techniques for treating a well bore in accordance with the invention, and various methods and devices may be employed to effect the necessary precautions in handling the source of the high intensity gamma radiation without departing from the scope of the invention. In essence, the invention consists in traversing a cased-in well bore with a source of gamma radiation of sufiicient intensity to destroy bacteria after passing through the well casing and into the subterranean formations surrounding the bore.

Other modes of applying the principle of our invention may be employed instead of those explained, change being made as regards the methods or materials employed provided the step or steps stated in any of the following claims, or the equivalent of such stated step or steps be employed.

We, therefore, particularly point out and distinctly claim as our invention:

1. The method of preventing bacterially-induced corrosion of well casings which comprises traversing the cased-in well bore with a source of gamma radiation. having a radiation intensity of at least about one curie per foot of source. 7

2. The method of claim 1 wherein the source of gamma radiation comprises a radioactive isotope having a radiation intensity of at least one curie per foot of source.

3. The method of claim 1 wherein the source of gamma radiation comprises uranium fission products having a radiation intensity of at least one curie per foot of source.

4. The method of claim 1 wherein the source of gamma radiation is passed through the cased-in bore at a rate between and about 6000 ft./hr.

References Cited in the file of this patent UNITED STATES PATENTS Craig Nov. 20, 1934 Pontecorvo May 23, 1950 OTHER REFERENCES 

